Expose AsyncIOWrapper publicly

[arian-bozorgzad]

This makes AsyncIOWrapper available as trio.AsyncIOWrapper, so it can be used to annotate values returned by open_file(), wrap_file(), and Path.open() without importing from the private _file_io module.

I added a short section to the docs as well, like including a note about using a protocol when only part of the async file interface is needed.

i also updated export test because AsyncIOWrapper has a slightly different static and runtime interface ,The test now checks that the difference is exactly the expected dynamic file methods before continuing with the normal export checks, and added type tests for open_file() and wrap_file().
and since AsyncIOWrapper exposes some file methods dynamically, I updated the export test to check that the difference between its static and runtime interfaces is exactly the expected set of dynamic methods.
Fixes #3390

python -m pytest src/trio/_tests/test_file_io.py
17 passed - 0 fails

python -m tox -e typing
mypy (linux): passed
mypy (darwin): passed
mypy (win32): passed
pyright: 0 errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (a3d7cdd) to head (8714b03).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3512   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19454        19461    +7     
  Branches         1321         1322    +1     
===============================================
+ Hits            19454        19461    +7     

Files with missing lines	Coverage Δ
src/trio/__init__.py	100.00000% <100.00000%> (ø)
src/trio/_file_io.py	100.00000% <100.00000%> (ø)
src/trio/_tests/test_exports.py	100.00000% <100.00000%> (ø)

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